Conventionally, bidirectional communication circuits as illustrated in FIGS. 9 and 10 are known as a communication device which transmits and receives a signal. The bidirectional communication circuit of FIG. 9 is provided with four signal terminals. In this communication circuit, a signal input from an input terminal 92h is processed by a level shift circuit G3 and the signal from the level shift circuit G3 is output to an output terminal 92k through a transistor T2, and a signal input from an input terminal 92i is processed by the level shift circuit G3 and the signal from the level shift circuit G3 is output to an output terminal 92j through a transistor T1.
On the other hand, the bidirectional communication circuit of FIG. 10 is provided with two signal terminals. In this communication circuit, a signal input from an input/output terminal 91c is processed by level shift circuits G1 and G2 and the signal from the level shift circuits G1 and G2 is output to an input/output terminal 91d through a transistor T2, and a signal input from the input/output terminal 91d is processed by the level shift circuits G1 and G2 and the signal from the level shift circuits G1 and G2 is output to the input/output terminal 91c through a transistor T1.
Although the input/output signal paths in the bidirectional communication circuit of FIG. 10 can be reduced in number from those in the bidirectional communication circuit of FIG. 9, the bidirectional communication circuit of FIG. 10 must be arranged to include malfunction preventive measures for preventing a malfunction due to congestion of the simultaneously incoming signals (for example, fixing the signals to H level or L level). Such malfunction preventive measures may be inclusion of a certain malfunction prevention circuit in the internal circuit units between the input/output terminals, or provision of restrictions in the communication protocols for inhibiting the bidirectional signals from being sent to the communication circuit at the same time.
With respect to this problem, Patent Document 1 listed below discloses a transmission/reception switching circuit which controls a transmitting circuit and a receiving circuit which are arranged in a communication device which transmits and receives a signal that is exchanged between a first input/output terminal and a second input/output terminal. In this transmission/reception switching circuit, a signal at the first input/output terminal and a signal at the second input/output terminal are detected, and one of operation of either the transmitting circuit or the receiving circuit is selectively enabled so that the transmitting operation by the transmitting circuit and the receiving operation by the receiving circuit do not conflict with each other.